Electrostatic developer composition containing both rough and smooth carrier particles

ABSTRACT

THIS INVENTION RELATES TO A PARTICULATE CARRIER VEHICLE AND A DRY ELECTROGRAPHIC DEVELOPER COMPOSITION CONTAINING SUCH CARRIER VEHICLE. THE PARTICULATE CARRIER VEHICLE DESCRIBED HEREIN CONTAINS A MIXTURE OF ROUGH-SURFACED MAGNETICALLY RESPONSIVE PARTICLES, E.G., SPONGE IRON POWDER AND SMOOTH-SURFACED MAGNETICALLY RESPONSIVE PARCLES, E.G., SMOOTH, SPHERICAL IRON PARTICLES.

w 174 w. TRA CHTENBERG ETAL ELECTROSTATIC DEVELOPER COMPOSITION CONTAINING BOTH ROUGH AND SMOOTH CARRIER PARTICLES Filed March v21, 19 2 2 Sheets-Sheet 1 H/GH RES/STANCE DEVELOPER RES/STANCE /0 4 LOW RES/STANCE sept. 24, 1974 w TRACHTENBERG ETAL 3,

ELECTROSTATIC DEVELOPER COMPOSITION CONTAINING BOTH ROUGH AND SMOOTH CARRIER PARTICLES Filed March 21 1972 2 Sheets-Sheet 2 DEVELOPER FLOWT/ME 5 (sec) WE/GHT 0F SMOOTH, CURV/L/A/EA/P PART/6255 //V CARR/E1? VEH/CLE F/GT C 3,838,054 ELECTROSTATIC DEVELOPER COMPOSITION CONTAINING BOTH ROUGH AND SMOOTH CARRIER PARTICLES William Trachtenberg, Rochester, John M. McCabe, Pittsford, and George P. Kasper, Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y.

Filed Mar. 21, 1972, Ser. No. 236,724 Int. Cl. G03g 9/02 US. Cl. 25262.1 12 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a particulate carrier vehicle and a dry electrographic developer composition containing such a carrier vehicle. The particulate carrier vehicle described herein contains a mixture of rough-surfaced magnetically responsive particles, e.g., sponge iron powder and smooth-surfaced magnetically responsive particles, e.g., smooth, spherical iron particles.

This invention relates to electrography and to a dry developer composition useful, for example, in the magnetic brush development of latent electrostatic charge images.

Electrographic imaging processes, especially electrophotographic processes and techniques, have been extensively described in both the patent and other literature, for example US. Pat. Nos.: 2,221,776, Calson, issued Nov. 19, 1940; 2,277,013, Carlson, issued Mar. 17, 1942; 2,297,691, Carlson, issued Oct. 6, 1942; 2,357,809, Carlson, issued Sept. 12, 1944; 2,551,582, Carlson, issued May 8, 1951; 2,825,814, Walkup, issued Mar. 4, 1958; 2,833,648, Walkup, issued May 6, 1958; 3,220,324, Snelling, issued Nov. 30, 1965; 3,220,831, McFarlane, issued Nov. 30, 1965; 3,220,833, McFarlane, issued Nov. 30, 1965 and many others. Generally, these electrophotographic processes have in common the steps of employing a normally insulating photoconductive element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming an electrostatic charge pattern. The electrostatic charge pattern is then rendered visible by a development step in which the charged surface of the photoconductive element is brought into contact with a suitable developer mix. One method for applying the developer mix is by the well-known magnetic brush process. Such a process generally utilizes apparatus of the type described, for example, in US. Pat. No. 3,003,- 462, Streich, issued Oct. 10, 1961, and customarily comprises a non-magnetic rotatably mounted cylinder having fixed magnetic means mounted inside. The cylinder isarranged to rotate so that part of the surface is immersed in or otherwise contacted with a supply of developer mix. The granular mass comprising the developer mix is magnetically attracted to the surface of the cylinder. As the developer mix comes within the influence of the field generated by the magnetic means within the cylinder, the particles thereof arrange themselves in bristle-like formations resembling a brush. The bristle formations of the developer mix tend to conform to the lines of magnetic flux, standing erect in the vicinity of the poles and laying substantially flat when said mix is outside the environment of the magnetic poles. Within one revolution the continually rotating tube picks up development mix from a supply source and returns part or all of this material to the supply. This mode of operation assures that fresh mix is always available to the copy sheet surface at its point of contact with the brush. In a typical rotational cycle, the roller performs the successive steps of developer mix pickup, brush formation, brush contact with the photoconductive element, brush collapse and finally mix release.

' United States Patent 3,838,054 Patented Sept. 24, 1974 In magnetic-brush development of electrostatic images the developer is commonly a triboelectric mixture of fine toner powder comprising dyed or pigmented thermoplastic resins with carrier particles of irregularly-shaped soft magnetic material such as ground chemical iron (iron filings) or reduced iron oxide particles. Several disadvantages relating to the use of rough, irregularly-surfaced magnetic carrier particles are generally recognized. First, the tendency of micro-size carrier particles to adhere to imaging surfaces is aggravated when the carrier surfaces are rough and irregular. In addition such rough and irregularly-surfaced particles undesirably tend to scratch a reusable electrophotographic surface during image transfer and surface cleaning operation. Moreover, unless these rough and irregularly-surfaced carrier particles are coated with an insulating material, a developer mix containing such irregularly-surfaced carrier particles results in poor reproductions of fine line images. On the other hand, when coated with a resinous material to provide an insulating surface, these rough, irregularly-surfaced particles do not exhibit as good flow properties through an electrographic developer system as would be desirable.

Recently it has been suggested that smooth, preferably curved-surfaced magnetic carrier particles, such as substantially spherical iron particles, or the like may, in certain cases, be preferable to the rough-surfaced carrier particles described above. This latter type of magnetic material, i.e. smooth-surfaced magnetic particles, has been found to provide generally good electrophotographic reproduction especially useful for reproducing fine-line and fuzzy carbon-paper images. In addition, this latter type of magnetic material, when used as a carrier in a magnetic-brush development composition, also provides good solid area reproduction. A more detailed description of this latter type of magnetic carrier material may be found in Canadian Pat. 838,061 issued Mar. 31, 1970. A method of making such smooth-surfaced magnetic particles may be found in US. Pat. 3,099,041 issued July 30, 1963. However, there are several problems related to the use of smooth-surfaced magnetic particles as carrier materials in a developer composition. A primary difiiculty encountered is the relatively high density of solid iron and similar ferromagnetic carrier materials which have these physical characteristics, i.e. smooth surfaces. Such high density, smooth particles tend to exhibit undesirable toner throwofi and difiicult toner replenishment characteristics. Furthermore, although such particles are quite dense and smooth, and accordingly flow extremely well in a magnetic brush developer system, these particles also have a relatively low apparent volume. Therefore, for optimum results, one generally uses about twice the weight level of these smooth-surfaced iron carrier particles in comparison to the weight of carrier required when one utilizes more conventional magnetic carriers such as the iron filings and reduced iron oxide particles, discussed hereinabove. The latter more conventional magnetic carrier particles are characterized by a much lower density and a much higher apparent volume. Because of the generally high weight level of smooth-surfaced iron particles which may be required to obtain an optimum apparent volume for use in magnetic brush development compositions, there is a much higher torque requirement on the rotatably mounted magnetic-brush cylinder. And, of course, because a greater weight of these smooth-surfaced iron carrier particles is required, significantly greater costs are involved.

In addition to the problems noted above relating to the use of certain magnetic carrier particles, the art acknowledges that it is generally known that electrostatic reproduction of originals, including those developed by the magnetic brush technique, which have large solid image areas thereon are susceptible to a defect known as edge effect. That is, the electroscopic toner particles are not uniformly deposited; most of the particles adhere to the edges of those solid areas leaving central zones of poor density. See, for example, U .S. Pat. 3,278,439 issued Oct. 11, 1966.

A further defect of many conventional electrostatic developer compositions relates to the mix life or the useful life of the developer mix. Mix life is defined herein to mean the total number of reproductions that may be produced by an electrographic system before the developer composition becomes inoperable, i.e. fatigued.

Numerous attempts have been made to correct the above-noted defects of electrostatic developers. For example, U.S. Pat. 2,965,573 issued May 2, 1958 describes a developer to which relatively large, filamentary electrically conductive particles are added to the developer apparently to correct edge effects. British Pat. 1,204,861 dated Sept. 9, 1970 describes a similar cascade development developer composition to which low density electrically conductive particles having a size preferably larger than the carrier particles have been added. In addition, Japanese patent publication No. 14,518/ 68 published June 19, 1968, describes an electrographic developer containing a ferromagnetic carrier composed of a mixture of electrically insulating resin coated iron powder and electrically conductive iron powder. A developer containing the latter type of mixed carrier is alleged to provide good electrographic reproductions of both fine line and solid area images.

In accordance with this invention there is provided an improved particulate carrier vehicle and an improved dry electrographic developer composition containing such a carrier vehicle. The particulate carrier vehicle of the invention comprises a mixture of rough-surfaced magnetically responsive particles and smooth-surfaced magnetically responsive particles. Preferably, the smooth-surfaced particles have rounded edges and curvilinear surfaces to provide enhanced flow through an electrographic development apparatus.

In accordance with the invention, the electrical properties of the carrier mixture may be varied to provide certain advantageous results. For example, in one embodiment of the invention the smooth-surfaced carrier particles may be modified to provide smooth-surfaced particles which have a relatively high or intermediate electrical resistance. According to this embodiment of the invention, it has been found that a carrier mixture comprising smoothsurfaced, electrically insulating magnetic particles and rough-surfaced, electrically conductive magnetic particles provides a useful carrier composition.

In another embodiment of the invention it has been found that a carrier mixture comprising rough-surfaced electrically insulating magnetic particles and smooth-surfaced electrically conducting magnetic particles provides a developer composition giving useful results.

In a preferred embodiment of the invention it has been found that a carrier composition comprising rough-surfaced electrically conductive magnetic particles and smooth-surfaced electrically conductive magnetic particles also provides a carrier composition yielding substantially improved results.

In still another embodiment of the invention it has been discovered that in carrier composition comprising roughsurfaced electrically insulating magnetic particles and smooth curvilinear-surfaced, electrically insulating magnetic particles provides a developer composition yielding useful results.

The advantages of the various embodiments of the invention described hereinabove are discussed in greater detail in the examples provided hereinafter. However, it can be generally noted that the carrier mixture and dry electrographic developers containing such mixtures provide at least one or more of the following advantages: improved image characteristics such as substantial reduction or elimination of edge-effect, increased mix life of the developer composition containing the carrier mixture of the invention, improved flow properties, reduced toner throw off i.e. reduction in the amount of toner thrown out of the developer composition as it flows through a magnetic brush development system, reduction in abrasion of the reusable photoconductive element used in an electrophotographic image system, ease of cleaning a photoconductive element used in an electrophotographic imaging system and ease of toner replenishment. In addition, it has been discovered that when a carrier mixture of the invention comprising a mixture of rough-surfaced, electrically conductive magnetic particles and smooth-surfaced electrically conductive carrier particles is utilized in an electrographic developer, one is able to obtain both good fringing and solid area development. This discovery is unexpected in view of the prior art teaching that low resistance carriers provide improved solid area coverage whereas high resistance carrier particles provide fine line (i.e. fringing) development.

FIGS. 1 and 2, as explained in further detail hereinafter in Examples 7 and 8, illustrate certain of the advantages provided by the invention. FIG. 1 illustrates the dependence of developer resistance on the composition and resistivity of the carrier vehicle. FIG. 2 illustrates the improved flow times obtainable using a developer containing a carrier vehicle comprising rough-surfaced and smooth-surfaced carrier particles.

To aid in identifying rough-surfaced magnetic particles and smooth-surfaced magnetic particles useful in the present invention, a particular sample of microsize magnetic particles may be viewed, for example, at x magnification. When viewed at 100 X magnification, magnetic particles having rough surfaces are readily distinguished from magnetic particles having smooth surfaces. For example, rough-surfaced particles when viewed at 100 magnification have one or more of the following characteristics: substantially larger surface area than smooth, curvilinearsurfaced particles sieved to the same mesh size; thin knife edges; pitted, fissured surfaces; bristle-like protrusions; irregular shapes.

On the other hand, the smooth surfaced particles when viewed at 100x magnification have the following characteristics: smooth, substantially continuous and substantially unbroken surfaces; generally curved surfaces, preferably with few visible edges or corners (if corners or edges are present, they typically appear as corners or edges of an equidimensional particle, e.g., a smooth-surfaced cubical particle, or they are rounded and blunt); preferably these particles have similar shapes, typically an oval or spherical shape, to promote gOOd flow properties. Accordingly, throughout the present specification and appended claims, the terms rough-surfaced and smooth-surfaced are defined in terms of the above-described 100 identirfication procedure.

The rough-surfaced magnetic particles of the present invention may be selected from a variety of magnetic particles having the aforementioned surface properties. Typically, these carrier particles have an average size within the range of from about 1200 microns to about 30 microns or less with particularly useful results being obtained with particles having an average size Within the range of from about 300 microns to about 60 microns. As used herein, the phrase average size refers to the average thickness of particles when measured along several axes. Average size also refers generally to the approximate size of the openings in a standard sieve series which will just retain or just pass a given particle. The phrase average size is thus not meant to imply that only perfectly uniform dimensioned particles can be used. The magnetic properties of these carrier particles may be imparted to the particles by a variety of techniques. These rough-surfaced magnetic particles may be formed from a non-magnetic core material such as organic resinous material or inorganic material such as a salt crystal or crystals, etc. having a magnetic overcoat applied thereto. Such a particle is described for example, in Belgian Pat. No. 726,806, published Mar. 14, 1969. Or these particles may have a magnetic core overcoated with a non-magnetic material such that the resultant particles have a rough surface. Still further, these rough-surfaced carrier'particles may be composed of a hollow magnetic ball having a rough surface. Preferably, however, these particles are solid ferromagnetic materials having rough surface characteristics. Such materials are well known in the electrographic art and are conventionally utilized in many magnetic brush development processes. Examples of such rough-surfaced particles are iron and'steel filings, sponge iron powder, etc. These materials are sold under various trade names such as SCM Corporation Glidden 475 electrolytic iron powder and Hoeganaes Corporation Ancor EH80-150 iron powder. Preferably, these rough-surfaced magnetic particles have a relatively low apparent density less than about 3.5 grams per milliliter, preferably on the order of from about 2 to about 3 grams per ml. Accordingly, in view of their low apparent density, these preferred particles typically may be characterized as porous, rough-surfaced particles.

Typically, the smooth-surfaced magnetic particles utilized in the present invention have an average size within the range of from about 1200 microns to about 30 microns or less with particularly useful results being obtained with particles having an average size within the range of from about 300 microns to about 60 microns. Although these particles are distinguished from the roughsurfaced magnetic particles described immediately hereinabove, it will be apparent that these smooth-surfaced particles may generally have a composition similar to the rough-surfaced particles. These smooth-surfaced particles may be composed of a non-magnetic core overcoated with a magnetic material such that the resultant particle has a smooth surface. Or, these particles may have a.,magnetic core overcoated with a non-magnetic material such that the resultant particles have a smooth surface. Still further, these particles may be composed of hollow magnetic balls having smooth surface characteristics. Preferably, however, these materials are solid ferromagnetic particles having a relatively high apparent density. Such solid particles typically, by themselves, have an apparent density greater than about 3.5 grams per ml.; preferably an apparent density within the range of from about 4.0 grams per ml. to about 5.0 grams per ml. Moreover, the magnetic remanance of these solid, smoothsurfaced magnetic iron particles is preferably significantly higher than the magnetic remanance of the preferred low density rough-surfaced iron or steel particles used in the carrier mixture. Advantageously, these smooth-surfaced iron particles have a magnetic remanance of about 8 or more maxwells. Especially preferred as the smooth-surfaced magnetic carrier particles are solid particles Which are substantially oval or spherical in shape and which are prepared by methods such as that described in U.S. Pat. 3,099,041, hereby incorporated in the present invention by reference thereto. Other useful smooth curvedsurfaced magnetic carrier particles and methods of preparing the same are described in Canadian Pat. 838,061.

Although solid ferromagnetic particles such as various iron, nickel, and cobalt compounds and alloys and mixtures thereof are preferred magnetic materials useful in the invention, other magnetically-responsive materials may be used. For example, a number of extremely small magnetically-responsive particles may be joined together by a binder material such as a resin to form a larger magnetic particle having the appropriate surface characteristics for use in the present invention. Such a carrier particle is described in Miller, Canadian Pat. 835,317 dated February 24, 1970.

The relative amounts of the rough-surfaced magnetic particles and the smooth-surfaced magnetic particles comprising the carrier mixtures of the present invention may vary widely. Typically, the carrier mixture should contain at least about 10% by weight of the rough-surfaced magnetic particles, and preferably from about -75% by weight of the rough-surfaced magnetic particles with the remainder of the carrier mixture comprising the smooth-surfaced magnetic particles. Although amounts somewhat less than or more than the aforementioned percentages may be utilized, it will be recognized that an amount much lower or much higher than those specified will lead to a carrier mixture which has nearly the same properties of a mixture composed solely of rough-surfaced particles or solely of smooth-surfaced particles and will not provide the advantages obtained using a mixture of these particles.

As noted hereinabove, several embodiments of the invention are provided herein whereby especially advantageous results may be obtained. Generally, these embodiments of the invention relate to changing the electrical properties of the particles comprising the carrier mixture of the present invention. For example, according to an especially preferred embodiment of the invention there is provided a mixture of electrically conductive smooth, curvilinear-surfaced magnetic particles and electrically conductive rough-surfaced carrier particles. According to this embodiment of the invention, it has been found that a carrier mixture comprising from about 25 to about 50 by weight of electrically conductive roughsurfaced magnetic particles and from about to about 50 by weight of electrically conductive, smooth, curvilinear-surfaced magnetic particles in admixture with a suitable electroscopic toner material provides a developer having certain definite advantages. In this regard, it has been found that addition of smooth, curvilinear-surfaced magnetic particles generally provides a developer having good flow properties (see FIG. 2 and Example 8) and a developer which gives good solid area images free from scratches, mottle, etc., and good reproduction of line images. On the other hand, it has been found that roughsurfaced magnetic carrier particles provides developers having good toner replenishment and toner throw-0E characteristics. Therefore, to optimize the advantageous features of both of these different types of carrier particles, the present invention calls for a mixture of the two. According to a preferred embodiment of the invention it has been found advantageous to maximize the amount of smooth, curvilinear-surfaced magnetic particles to produce the desired image characteristics while maintaining the good replenishment and throw-off characteristics associated with rough-surfaced magnetic particles. It has been found that for a given developer bulk resistance, a maximum of smooth-surfaced magnetic particles in a developer mixture is obtained when both the rough-surfaced and the smooth-surfaced particles have a relatively low electrical resistivity. (See FIG. 1 and Example 7.) Thus, in a preferred embodiment of the invention, as noted above, there is provided a developer comprising a carrier vehicle containing a mixture of electrically conducting smooth, curvilinear-surfaced and rough-surfaced magnetic particles.

Although, as stated above, a preferred embodiment of the present invention relates to a carrier comprising a mixture of electrically conducting smooth-surfaced and rough-surfaced magnetic particles, it will be appreciated that the present invention is not limited thereto. For example, in certain situations, one may desire a fringing developer only. In such case the electrical properties of the developer mixture of the present invention may be varied to include electrically insulating particles therein. According to other embodiments of the invention one may "utilize a carrier mixture comprising electrically insulating smooth-surfaced and rough-surfaced magnetic particles, or a mixture of electrically insulating smooth-surfaced magnetic particles and electrically conductive rough-surfaced magnetic particles, or a mixture of electrically conducting smooth-surfaced magnetic particles and electrically insulating rough-surfaced magnetic particles.

In the present specification, the terms electrically insulating and electrically conducting are defined herein as follows: electrically insulating carrier particles or developer mixes (i.e., mixtures of carrier particles and dry toner powder) have an electrical resistance greater than about ohms up to 10 ohms and higher whereas electrically conducting carrier particles or developer mixes are defined as having a lower electrical resistance, i.e., less than about 10 ohms down to 10 ohms or lower. The electrical resistance of carrier particles and developer mixes as defined herein is measured according to the following test: A 15 gram quantity of the particular carrier particles or developer mix to be tested is selected. A cylindrically-shaped bar magnet having a circular end of about 6.25 square cm. in area is used to attract the 15 grams of sample carrier and hold it in the form of a brush. After formation of the brush, the bar magnet is positioned with the brush-carrying end approximately parallel to "and about 0.5 cm. from a burnished copper plate. The

resistance of the particles in the magnetic brush is then measured between the magnet and the copper plate.

A variety of well known techniques are available and known in the art for modifying the electrical surface properties of carrier particles. Accordingly, extended discussion thereof is not necessary. Brief mention may be made of various methods of coating electrically insulating resins on carrier particles to impart electrically insulating properties thereto and methods for improving the electrical conductivity of various types of carrier particles. In this regard, reference may be made to the following publications: Walkup, US. Pat. 2,618,551 issued Nov. 18, 1952; Greig, US. Pat. 2,874,063 issued Feb. 17, 1959; Belgian Pat. No. 746,111 dated Apr. 30, 1970; Belgian Pat. No. 746,110 dated Apr. 30, 1970; Belgian Pat. No. 746,109 dated Apr. 30, 1970; Miller, US. Pat. 3,547,822 issued Dec. 15, 1970; Canadian Pat. No. 835,817 dated Feb. 24, 1970; and Hagenbach et al. US. Pat. 3,533,835 issued Oct. 13, 1970.

Electrostatic developer compositions of the present invention can be prepared by mixing from about 90 to about 99% by weight of the above-described carrier vehicle with from about 10 to about 1% by weight of a suitable electroscopic toner material.

The toner material (or marking particles) useful in dry electrographic developer compositions are generally comprised of a resin binder and a colorant. Suitable toners can be selected from a wide variety of materials to give desired physical properties to the developed image and the proper triboelectric relationship to match the carrier particles used. Generally, any of the toner powders known in the art are suitable for mixing in the developer composition of this invention. In certain instances, the toner may be comprised solely of colorant material without any resinous binder. In other cases, where a visible image is not desired or needed, the toner may be composed solely of a colorless material, such as a resinous material, having the desired physical and triboelectric properties.

When the toner powder selected is utilized with magnetic carrier particles in a magnetic-brush development arrangement, the toner clings to the carrier by triboelectric attraction. The carrier particles acquire a charge of one polarity and the toner acquires a charge of the opposite polarity. Thus, if the carrier is mixed with a resin toner which is higher in the triboelectric series, the toner normally acquires a positive charge and the carrier a negative charge.

Useful toner particles can be prepared by various methods. Two convenient techniques for preparing these toners are spray-drying or melt-blending followed by grinding. Spray-drying involves dissolving the resin, colorant and any additives in a volatile organic solvent such as dichloromethane. This solution is then sprayed through an atomizing nozzle using a substantially non-reactive gas such as nitrogen as the atomizing agent. During atomization, the volatile solvent evaporates from the airborne droplets, producing toner particles of the uniformly colored resin. The ultimate particle size is determined by varying the size of the atomizing nozzle and the pressure of the gaseous atomizing agent. Further details relating to spraydrying may be found in Carlson, U.S. Pat. 2,357,809 issued Sept. 12, 1944, and in West, US. Pat. 3,166,510 issued I an. 19, 1965. conventionally, particles of a diameter between about /2 i and about 30 are used, with particles between about 2p. and 15 being preferred, although larger or smaller particles can be used Where desired for particular development or image considerations.

Suitable toners can also be prepared by melt-blending. This technique involves melting a powdered form of polymer or resin and mixing it with suitable colorants and additives. The resin can readily be melted or heated on compounding rolls which are also useful to mix or otherwise blend the resin and addenda so as to promote the complete intermixing of these various ingredients. After thorough blending, the mixture is cooled and solidified. The resultant solid mass is then broken into small pieces and finely ground to form a free-flowing powder of toner particles. Such a melt-blending technique is described in Walkup, US. Pat. 2,618,551 issued Nov. 18, 1951. Of course, various other techniques for making toner particles may also be used. For example, certain spray-freeze drying techniques may be modified to provide useful methods for preparing toner particles. An example of such a modified spray freeze drying technique is described in Product Licensing Index, Volume 84, Apr. 1971, pages 343 6. The resultant toner particles usually range in size from about A: to about 30a.

The resin material used in preparing the toner can be selected from a wide variety of materials, including natural resins, modified natural resins and synthetic resins. Exemplary of useful resins are balsam resins, colophony, and shellac. Exemplary of suitable modified natural resins are colophonyl-modified phenol resins and other resins listed below with a large proportion of colophony. Suitable synthetic resins are all synthetic resins known to be useful for toner purposes, for example, polymers, such as certain polycarbonate resins described in US. patent application Ser. No. 34,557 filed May 4, 1970, and in Product Licensing Index, Volume 84, April 1971, pages 69-70, vinyl polymers and copolymers including polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, polyvinyl acetals, polyvinyl ether, polyacrylic and polymethacrylic esters, polystyrene, including substituted polystyrenes; polycondensates, e.g., polyesters, such as phthalate, terephthalic and isophthalic polyesters, maleinate resins and colophony-mixed esters of higher alcohols; phenolformaldehyde resins, including modified phenol-formaldehyde condensates; aldehyde resins; ketone resins; polyamides; polyurethanes, etc. Moreover, chlorinated rubber and polyolefins, such as various polyethylenes, polypropylenes, polyisobutylene, are also suitable. Typical toner materials having incorporated therein styrene containing materials are disclosed in the following US. Patents: Solar 2,917,- 460 issued Dec. 15, 1959; Carlson Re. 25,136 issued Mar. 13,1962; Rheinfrank et al. 2,788,288 issued Apr. 9, 1957; Walkup et al. 2,638,416 issued Apr. 12, 1953; Wise 2,618,- 552 issued Nov. 18, 1952; and Copley 2,659,670 issued Nov. 17, 1953. Other useful styrene containing toner resins are copolymers prepared from a monomeric blend of (a) 40-60% by weight styrene or styrene homolog; (b) 20- 50% by weight of lower alkyl acrylate and methacrylate, e.g. alkyl methacrylates and alkyl acrylates having up to 3 carbon atoms in the alkyl group; and (0) 530% by weight of higher alkyl acrylates and methacrylates, e.g. alkyl methacrylates and alkyl acrylates having 6-20 or more carbon atoms in the alkyl group.

Colorants useful in the practice of this invention can be selected from a variety of materials such as dyestuffs or pigments. Such materials serve to color the toner and thus render it more visible. Suitable toner materials having appropriate caking and charging properties can, of course, be prepared without the use of a colorant material Where it is desired to have a developed image of low optical opacity. In those instances where it is desired to have high optical opacity, the colorants usedcan, in principle, be selected from virtually all of the compounds mentioned in the Color Index, Vols. I and II, Second Edition, Included among the vast number of useful colorants would be such materials as Hansa Yellow G (C.I. 11680), Nigrosine Spirit soluble (C.I. 50415), Chromogen Black ETOO (CI. 14645), Rhodamine B (C.I. 45170), Solvent Black 3 (Cl. 26150), Fuchsine N (C.I. 42510), C.I. Basic Blue 9 ((3.1. 52015), etc. Another useful class of colorants is comprised of nigrosine salts such as nigrosine salts of monoand (ii-functional organic acids having from about 2 to about 20 carbon atoms such as chloroacetic acid, stearic acid, sebacic acid, lauric acid, azelaic acid, adipic acid, abietic acid and the like. Nigrosine salts of this type are disclosed in Belgian Pat. 734,570 dated Aug. 14, 1969. Carbon black also provides a useful colorant as disclosed in Walkup, US. Pat. 2,618,551 issued Nov. 18, 1952. The amount of colorant added may vary over a wide range, for example, from about 3 to about 20 percent of the weight of the resin binder. Particularly goOd results are obtained when the amount is from about 5 to about percent. In certain instances, it may be desirable or preferred to omit the colorant, in which case the lower limit of concentration would be zero.

In addition, if desired, organic melt viscosity modifying agents for the resin binder may be incorporated in the toner particles. If such viscosity modifying agents are utilized, the ratio of melt viscosity modifying agent to resin binder generally can vary from about 1:3 to about 1.5: 1. Further information concerning these viscosity modifying agents may be found in Merrill et al., US. Application Ser. No. 23,031, filed Mar. 26, 1970.

The following examples further illustrate the invention.

The following definitions are utilized in the examples.

H: Refers to Hoeganaes Corp. Ancor EH80-150 sponge iron powder, a rough-surfaced powder sieved to 80 +120 mesh.

W: Refers to high density, smooth-surfaced spherical iron powder particles obtained from the Nuclear Metals Division of Whittaker Corp. having an average particle size within the sieve range of 70 +170 mesh and an apparent density of about 4.64 g./ml.

Conducting: Refers to treatment of the magnetic particle with an acid wash as described in Belgian 746,109 dated Apr. 30, 1970, followed by nickel plating as described, for example, in Belgian 746,110 dated Apr. 30, 1970, to provide particles which are electrically conducting as defined hereinbefore.

Insulating: Refers to the coating of the magnetic particles with a thin layer of a polymeric material such as a vinylidene chloride-acrylonitrile-acrylic acid terpolymer or a metal ion-linked carboxylic acid-a-olefin copolymer (such as those sold under the trademark of Elvax dispersions by E. I. du Pont & Co.) to provide particles which are electrically insulating as defined hereinbefore. The use of such materials are described in McCabe, patent application Ser. No. 236,765 filed concurrently herewith and entitled Electrographic Carrier Vehicle and Developer CompositionCase B and in Kasper, patent application Ser. No. 236,584, filed concurrently herewith and now abandoned and entitled Electrographic Carrier Vehicle and Developer Composition-Case C.

The terms relating to image type are used in the accompanying Examples as follows:

SA means solid area development predominates. That is, one obtains good copies of solid area images including good fill-in of solid area images.

S-S means semisolid area development predominates characterized by slight loss in density of the center of a copy of a 25 mm. square solid image.

M means medium area development predominates characterized by a large loss in density of the center of a copy of a 25 mm. square solid image.

S-F means semi-fringed development predominates wherein little density is produced in the center of a copy of a 25 mm. square solid image.

F means fringed development predominates wherein no density is produced over the majority of the internal area of a copy of a 25 mm. square solid image.

Line Reproduction: Refers to the ability to reproduce fine, weak and unsharp lines.

smoothness: Refers to the ability to reproduce solid area images free from scratches, mottle, etc.

Throwofi: Refers to the developers ability to hold toner without dusting as the developer is mechanically agitated.

Replenishment: Refers to the developers ability to accept toner in the replenishment mode.

Flow: Refers to the developers ability to be carried around the magnetic brush during development.

P,A,S: Refer to the ratings of poor, acceptable and superior, respectively.

Example 1 Part 1 of Table I, included below for purposes of comparison, lists the data which are obtained when a carrier vehicle containing a mixture of rough-surfaced electrically insulating particles and rough-surfaced electrically conducting particles is used. Note particularly that the weakness and strengths of the mixtures are controlled by the individual carrier components. A carrier mixture of this type exhibits three defects: poor fine-line reproduction, poor quality solid-area reproduction, and poor flow characteristics. This rough-surfaced carrier material appears to have properties which prevent it from perforning satisfactorily; e.g., rough-surfaced sponge iron has a craggy surface which is rough and irregular. Thus particles of this type are highly abrasive during the development operation causing poor fine-line reproduction, scratchy solid areas and characters, and poor developer flow.

The surface properties of these rough-surfaced particles, however, do offer some distinct advantages. Because their surface area is large and rough-textured, these carrier particles will hold toner extremely well, producing very favorable throwoff and replenishment characteristics. In addition, they have a scrubbing action which tends to keep the level of background down, producing very clean prints. Carrier A, in this example is H-Conducting. Carrier B, in this example is H-Insulating, having a thin external layer composed of vinylidene chloride-acrylonitrileacrylic acid copolymer.

Example 2 Part 2 of Table I included for purposes of comparison shows that both conducting and insulating smooth-surfaced iron particles are acceptable in all imaging characteristics, but fall short with regard to throwolf and replenishment. The iron particles used in this example are quite dense, spherical and extremely smooth, quite different from the particles used in Example 1. The electrographic properties of these smooth-surfaced particles are also different from those exhibited by the particles used in Example 1. If the insulating and conducting particles used in this example are mixed together, the carrier mixture is at best, a poor compromise. Carrier A in this example is W-Conducting. Carrier B in this example is W-Insulating,

having a thin external layer composed of a vinylidene chloride-acrylonitrile-acrylic acid copolymer.

Example 3 Part 3 of Table I shows the results obtained when smooth-surfaced particles are mixed with rough-surfaced particles in accordance with the present invention. In order for a mixture to be judged acceptable in all areas, it should have an A or S rating in every category.

Thus, the more A+S ratings a particular carrier vehicle is given, the more useful the carrier vehicle becomes.

The data of Part 3 indicate that a mixed carrier vehicle of the invention satisfies all categories. The smoothsurfaced particles contribute to the imaging properties to the extent that acceptable mages are generated, despite the presence of the rough-surfaced particles. The roughsurfaced particles provide the necessary physical properties to hold the toner in the developer bulk. Carrier A in this example is H-Conducting. Carrier B in this example is W-Insulating, having a thin external layer composed of a vinylidene chloride-acrylonitrile-acrylic acid copolymer.

Example 4 Part 4 of the table again shows the improved results obtained by the use of the mixed carrier of this invention. This example differs from Example 3 in that the rough, irregularly-surfaced iron is overcoated with a polymer to make it electrically insulating and the smooth-surfaced iron is nickeled to make it more conducting. The table indicates that the insulating rough-surfaced carrier by itself satisfies most of the tested requirements. This is to be expected, since the overcoat material may, to an extent, fill up the interstitial voids in the carrier rendering it more round and smooth. But the rounding and smoothing operation does not convert such particles completely to smooth-surfaced particles. The insulating rough-surfaced particles have at least one weakness which prevents a carrier of these particles from being totally accepted. That is, such a carrier provides primarily fringing and semifringing developed images apparently due to the infiuence of the insulating polymer coating and does not give good reproduction of solid area images. To obtain good solid-area reproduction, such a carrier should be admixed with a solid-area carrier, in this case conducting smoothsurfaced iron particles. The admixture of smooth-surfaced iron particles also improves the flow properties of the carrier.

The table shows that such a mixture works extremely well as 50/50, 67/33 and 75/25 ratios.

Carrier A in this example is H-Insulating having a thin external layer composed of a vinylidene chloride-acrylo nitrile-acrylic acid copolymer. Carrier B in this example is W-Conducting.

Example 5 Part 5 of the table shows that this invention is not restricted to the use of insulating/conducting carrier mixtures only. When a smooth-surfaced conducting carrier and a rough-surfaced conductive carrier are mixed together, the superior properties of each blend together to produce mixtures which are acceptable in every category. Carrier A is H-Conducting. Carrier B is W-Conducting. This Example illustrates a preferred embodiment of the invention as explained in greater detail hereinbefore.

Example 6 Part 6 of the table indicates that the advantages of the invention can be realized by a mixture of two insulating mixtures. Note in particular the superior performance of the mixtures in the ratios of 40/60 and 35/65. Carrier A is H-Insulating, having a thin outer layer composed of a dried Elvax resinous dispersion, said outer layer containing about 33% by weight of electrically conductive carbon particles having a particle size within the range of 1-20 microns. Carrier B is W-Insulating, having a thin external layer composed of a dried Elvax resinous dispersion, said outer layer containing about 35% by weight of small electrically conductive carbon particles having a particle size within the range of 1-20 microns.

Example 7 Table I shows that flow, smoothness and line reproduction are improved by the use of a smooth particle.

12 Likewise, replenishment and throwoif are improved by the use of a rough particle. It is advantageous, therefore, to add a maximum of smooth carrier component to produce the desired image while maintaining good replenishment and throwoff characteristics.

FIG. 1 of this example illustrates that for a given developer resistivity a maximum of smooth carrier component in a mixture is obtained when both particles are of low resistivity. The materials used to obtain the data on which FIG. 1 is based are described below as follows:

The low resistivity carrier particles used in FIG. 1 are prepared by utilizing ferromagnetic particles having an oxide surface such that the electrical resistance of the particles is less than about 10 ohms using the resistance test described hereinbefore. The polymer coating is composed of a dried Elvax resinous dispersion as described hereinbefore. The higher resistance carrier particles used in FIG. 1 of this example are prepared in a manner such that the oxide surface on the ferromagnetic particles provides particles having a resistance greater than about 10 ohms. In FIG. 1 of this example, carrier A is composed of W iron particles having an Elvax coating thereon and has a relatively high electrical resistance. Carrier B is composed of H iron particles having an Elvax coating thereon and has a relatively low resistance. Carrier C is composed of W iron'particles having an Elvax coating thereon and has a relatively low electrical resistance. In the graph of FIG. 1, the horizontal axis represents the weight percent of smooth curvilinear-surfaced iron particles, i.e., W iron particles, present in a carrier vehicle composed of a mixture of smooth and rough-surfaced particles. The vertical axis of the graph represents the electrical resistance of a developer containing 3-5 by weight toner and -97% by weight of the carrier vehicles represented on the horizontal axis. The electrical resistance of a given developer is determined as indicated earlier in the present specification.

The dry developers used in Examples 1-7 above contain from about 3 percent to about 5 percent by weight of toner which is composed of particles of a black pigmented polycarbonate resin, said resin particles described in US. patent application Ser. No. 34,557, filed May 4, 1970, and Product Licensing Index, Volume 84, April 1971, pages 69-70.

Example 8 FIG. 2 based on this example illustrates the improvement in developer flow achieved by the addition of smooth, curvilinear-surfaced carrier particles to a carrier vehicle composed of a mixture of smooth and rough-surfaced particles. In this example, the data on which FIG. 2 is based is obtained using W-iron particles having an Elvax coating as the smooth, curvilinear-surfaced particles and H-iron particles having an Elvax coating as the roughsurfaced iron particles. In the graph of FIG. 2, the horizontal axis represents the weight percent of smooth, curvilinear-surfaced iron particles, i.e., W iron particles, present in a carrier vehicle composed of a mixture of smooth and rough-surfaced, i.e., H-iron particles. The vertical axisof the graph of FIG. 2 represents the flowtime in seconds for a unit volume of developer. Flowtimes are determined as follows: Using a Hall Flowmeter, sufiicient developer (toner plus carrier) is allowed to flow through a standard Hall Flowmeter cup #856 (rated at 39.9 seconds) mounted one inch above a 25 ml. receiving cup until the cup is completely filled (25 ml.). The developer is then placed back in the standard cup #856 and the time (in seconds) for all of the developer to flow through the standard orifice is measured as the flowtime. Each of the developers used to provide the data for the FIG. 2 graph are composed of 97% by weight carrier particles and 3% by weight of pigmented thermoplastic resin toner particles.

TABLE I Line reproduction ness Mixture (weight Image percent) Conducting. It

mwmm b i-q m p mmmmmm m to wmmg g m H) *d m m ppwi-g C mmmbm l-d mwm Pd mmmmmmm p. thrombi-grew l-d m m *drdtdwp,

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

We Claim:

7 1 An electrographic developer composition, for use in developing electrostatic charge patterns, comprising a free flowing particulate, magnetically-responsive carrier vehicle and particulate electroscopic toner material triboelectric'ally -attractable to said carrier vehicle, said carrier vehicle comprising a mixture of rough-surfaced, irregularly shaped magnetically responsive particles and smooth-surfaced, regularly shaped magnetically responsive particles, said rough and said smooth particles having an average size within the range of from about 1200 microns to about 30 microns, said rough particles being present in an amount of from about percent to about 83 percent by weight of the carrier vehicle.

2. The invention of claim 1 wherein said magneticallyr'e sponsive particles comprise solid particles of ferromagnetic materials selected from the group consisting of iron, cobalt, nickel and alloys and mixtures thereof.

3'. The invention of claim 1 wherein said'rough-surfaced; irregularly shaped magnetically-responsive particles are electrically conducting and said smooth-surfaced, regularlyshaped magnetically-responsive particles are electrically insulating.

4. The invention of claim 1 wherein said rough-surfaced magnetically-responsive particles are electrically insulating and said smooth-surfaced particles are electrically conducting.

5; The invention of claim 1 wherein said rough and said smoothparticles are electrically insulating.

Smooth- Replen- Throwofi ishment Flow 6. A11 electrographic developer composition, for use in developing electrostatic charge patterns, comprising a free flowing particulate, magnetically-responsive carrier vehicle and particulate electroscopic toner material triboelectrically-attractable to said carrier vehicle, said carrier vehicle comprising a mixture of rough-surfaced, irregularly shaped magnetically responsive particles and smoothsurfaced, regularly shaped magnetically responsive particles, said rough and said smooth particles having an average size within the range of from about 1200 microns to about 30 microns, said rough and said smooth particles being electrically conductive, and said rough particles being present in an amount of from about 10 percent to about 83 percent by weight of the carrier vehicle.

7. An electrographic developer composition, for use in developing electrostatic charge patterns, comprising a free-flowing particulate, magnetically-responsive carrier vehicle and particulate electroscopic toner material triboelectrically-attractable to said carrier vehicle, said carrier vehicle comprising a mixture of rough-surfaced, irregularly shaped ferromagnetic particles having an apparent density less than about 3.5 g./ml. and smooth, regularly shaped convexly-surfaced ferromagnetic particles having an apparent density greater than about 3.5 g./ml., said rough and said smooth particles having an average size within the range of from about 1200 microns to about 30 microns, said rough particles being present in an amount of about 10 percent to about 83 percent by weight of the carrier vehicle.

8. The invention of claim 7 wherein said rough and said smooth particles comprise iron and wherein said rough and said smooth particles have an average size within the range of from about 300 microns to about 60 microns.

9. An electrographic developer composition, for use in developing electrostatic charge patterns, comprising a free flowing particulate, magnetically-responsive carrier vehicle and particulate electroscopic toner material triboelectrically-attractable to said carrier vehicle, said carrier vehicle comprising a mixture of (a) rough-surfaced, irregularly shaped, electrically conducting ferromagnetic iron or iron alloy particles having an apparent density within the range of from about 2 to about 3 g./ml. and (b) smooth, regularly shaped, electrically conducting, substantially spherical, ferromagnetic iron or iron alloy particles having an apparent density greater than about 4 g./ml., said rough and said smooth particles having an average size within the range of from about 60 to about 300 microns, said rough particles being present in an amount of from about percent to about 83 percent by weight of the carrier vehicle.

10. The invention of claim 9 wherein said rough and said smooth particles have an electrical resistance less than about 10 ohms.

11. An electrographic developer composition, for use in developing electrostatic charge patterns, comprising a free flowing particulate, magnetically-responsive carrier vehicle and particulate electroscopic toner material triboelectrically-attractable to said carrier vehicle, said carrier vehicle comprising a mixture of rough-surfaced, irregularly shaped magnetically responsive particles and smoothsurfaced, regularly shaped magnetically responsive particles, said rough and said smooth particles having an average size within the range of from about 1200 microns to about 30 microns, said rough and said smooth particles being electrically conductive, and said rough particles being present in an amount of from about percent to about 75 percent by weight of the carrier vehicle.

12. An electrographic developer composition, for use in developing electrostatic charge patterns, comprising a free flowing particulate, magnetically-responsive carrier vehicle and particulate electroscopic toner material triboelectrically-attractable to said carrier vehicle, said carrier vehicle comprising a mixture of (a) rough-surfaced, irregularly shaped, electrically conducting ferromagnetic iron or iron alloy particles having an apparent density within the range of from about 2 to about 3 g./ml. and (b) smooth, regularly shaped, electrically conducting, substantially spherical, ferromagnetic iron or iron alloy particles having an apparent density greater than about 4 g./ ml., said rough and said smooth particles having an average size within the range of from about to about 300 microns, said rough particles being present in an amount of from about 15 percent to about percent by weight of the carrier vehicle.

References Cited UNITED STATES PATENTS 3,406,105 10/1968 Letendre 252513 3,627,682 12/1971 Hall et al. 117235 3,278,439 10/1966 Blanchette et al. 252-62.1 3,165,420 1/1965 Tomanek et al 25262.1 3,015,305 1/1962 Hall et al. 25262.1 2,965,573 12/1960 Gundlach 25262.1 2,874,063 2/1959 Grey 252--62.1

FOREIGN PATENTS 1,174,571 12/1969 Great Britain 252-62.1

RONALD H. SMITH, Primary Examiner J. P. BRAMMER, Assistant Examiner US. Cl. X.R.

rzggg UNITED STATES PATENT OFFICE v CERTIFICATE OF CORRECTION Fate Nah U.S. 3,838,05M Dam]- September 24, 197A W. Trachtenberg et. a1.

- It is certified that error appears in the above-idehtified patent and that said Letters Patent are hereby corrected as shown below:

Col. 6, line'23,"'50"-should read +50%", M e a line 25, "50" should read '--50%--.

Col. 11, line 6, "mages" should read "images-" Signed and sealed this 7th day of January 1975.

(Sm-u)- Attest':' I I-icCO e1. .5153051 JR. v c Z' -iIARSHALL. DANN Attesting Officer Commissioner of Patents 

